This invention relates to the recovery of oil from subterranean oil reservoirs and more particularly to improved waterflooding operations involving the injection of aqueous surfactant solutions.
In the recovery of oil from oil-bearing reservoirs, it usually is possible to recover only minor portions of the original oil in place by the so-called primary recovery methods which utilize only the natural forces present in the reservoir. Thus, a variety of supplemental recovery techniques have been employed in order to increase the recovery of oil from subterranean reservoirs. The most widely used supplemental recovery technique is waterflooding, which involves the injection of water into the reservoir. As the water moves through the reservoir, it acts to displace oil therein to a production system composed of one or more wells through which the oil is recovered.
It has long been recognized that factors such as the interfacial tension between the injected water and the reservoir oil, the relative mobilities of the reservoir oil and injected water, and the wettability characteristics of the rock surfaces within the reservoir are factors which influence the amount of oil recovered by waterflooding. Thus, it has been proposed to add surfactants to the flood water in order to lower the oil-water interfacial tension and/or to alter the wettability characteristics of the reservoir rock. Also, it has been proposed to add viscosifiers such as polymeric thickening agents to all or part of the injected water in order to increase the viscosity thereof, thus decreasing the mobility ratio between the injected water and oil and improving the sweep efficiency of the waterflood.
Processes which involve the injection of aqueous surfactant solutions are commonly referred to as surfactant waterflooding or as low tension waterflooding, the latter term having reference to the mechanism involving the reduction of the oil-water interfacial tension. Thus far, many such waterflooding applications have employed anionic surfactants. For example, a paper by W. R. Foster entitled "A Low-Tension Waterflooding Process", Journal of Petroleum Technology, Vol. 25, Feb. 1973, pp. 205-210, describes a promising technique involving the injection of an aqueous solution of petroleum sulfonates within designated equivalent weight ranges and under controlled conditions of salinity. The petroleum sulfonate slug is followed by a thickened water slug which contains a viscosifier such as a water-soluble biopolymer in a graded concentration in order to provide a maximum viscosity greater than the viscosity of the reservoir oil and a terminal viscosity near that of water. This thickened water slug is then followed by a driving fluid such as a field brine which is injected as necessary to carry the process to conclusion.
One problem encountered in waterflooding with certain of the anionic surfactants such as the petroleum sulfonates is the lack of stability of these surfactants in a so-called "high brine" environment. These surfactants tend to precipitate from solution in the presence of monovalent salts such as sodium chloride in concentrations in excess of about 2 to 3 weight percent and in the presence of much lower concentrations of divalent metal ions such as calcium and magnesium ions. Typically, divalent metal ion concentrations of about 50 to 100 ppm and above cause precipitation of the petroleum sulfonates. The salinity of the surfactant slug is also significant with regard to interfacial tensions achieved through the use of petroleum sulfonates such as disclosed in the Foster paper. Even in the absence of divalent metal ions, optimum interfacial tensions are seldom achieved at salinities significantly in excess of 2 to 3 weight percent.
Various surfactant formulations which will tolerate high salinities and/or high divalent metal concentrations have been proposed for use in high brine environments. Thus, U.S. Pat. No. 3,508,612 to Reisberg et al. discloses a low tension waterflooding process employing a calcium-compatible anionic-nonionic surfactant system which can be employed in saline solutions containing from 0.01 to 5 molar NaCl and from about 0 to 0.1 molar CaCl.sub.2. One of the anionic surfactants employed in the Reisberg et al. process is an organic sulfonate such as a petroleum sulfonate having an average molecular weight within the range of 430-470 and the other surfactant is a sulfated ethoxylated alcohol.
Another waterflooding technique involving the use of calcium-compatible surfactant systems is disclosed in U.S. Pat. No. 3,827,497 to Dycus et al. In this process, a three-component or two-component surfactant system may be employed. The three-component system comprises an organic sulfonate surfactant such as a petroleum sulfonate, a polyalkylene glycol alkyl ether, and a salt of a sulfonated or sulfated oxyalkylated alcohol. The two-component system comprises an organic sulfonate surfactant and a salt of a sulfonated oxyalkylated alcohol. These surfactant systems may be employed in a brine solution which, as noted in column 3, will usually contain about 0.05-8 percent sodium chloride and will often contain 50-5,000 ppm polyvalent metal ions such as calcium and/or magnesium ions. The sulfated or sulfonated oxyalkylated alcohols may be derived from aliphatic alcohols of 8-20 carbon atoms or from alkyl phenols containing 5-20 carbon atoms per alkyl group. The oxyalkyl moiety in this surfactant will usually be derived from ethylene oxide although other lower alkylene oxides containing 2-6 carbon atoms or mixtures thereof may be employed.
Another surfactant waterflooding process for use in high salinity environments is disclosed in U.S. Pat. No. 3,977,471 to Gale et al. This patent discloses the use of an R.sub.1 hydrocarbyl ether-linked R.sub.2 hydrocarbyl sulfonate. The R.sub.1 lipophilic base is provided by a benzene, toluene, or xylene radical having an alkyl substituent containing 6-24 carbon atoms and the R.sub.2 linking the sulfonate group with the alkoxy ether group is a C.sub.1 -C.sub.8 alkyl, cycloalkyl, alkene or aryl radical. The R.sub.2 hydrocarbyl group may be substituted with a hydroxy group or a C.sub.1 -C.sub.8 aliphatic group. Dodecyl, dimethyl benzene ether propane sulfonate is specifically disclosed by Gale et al. The Gale et al. process is said to be particularly useful in reservoirs having high salinity brines, i.e., salinities of 2% or more, and the dodecyl, dimethyl benzene ether propane sulfonate is said to be stable in saline solutions containing from 7-14 weight percent sodium chloride.
Yet another surfactant waterflooding process for use in which brine environments is disclosed in U.S. Pat. No. 4,008,768 to Birk. This patent discloses an anionic-nonionic surfactant system suitable for use in aqueous environments exhibiting salinities within the range of 3-25 weight percent and containing divalent metal ions in amounts as high as 3 weight percent. A multicomponent surfactant system disclosed for use in the Birk process comprises an amide-linked sulfonate such as a di-substituted taurate containing a C.sub.12 -C.sub.20 aliphatic group and a nonionic surfactant such as a polyethoxylated alkyl phenol or a polyethoxylated aliphatic alcohol.